The objective of the proposed research is to increase our understanding of the synaptic mechanisms used by local circuit neurons and, in particular, to study the function of graded synaptic transmission (GST) in spiking, local circuit neurons. A comparative study between identified neurons of two different preparations (the stomatogastric ganglion of spiny lobster and the buccal and abdominal ganglia of Aplysia) will use intracellular recording and voltage clamp techniques to describe neuron-to-neuron GST behavior and to compare it to the behavior of spike-evoked postsynaptic potentials (PSPs) in the same neurons. A detailed study of the mechanisms underlying GST and a comparison to those of PSPs will focus on voltage-dependent ionic currents (e.g. presynaptic calcium current) and neuronal shape by using the techniques of voltage clamp, neuroanatomy, cable analysis, and drugs. Special attention will be given to the plastic properties of synapses, including time dependent effects and modulation by inputs to the presynaptic neuron (e.g. presynaptic inhibition). Such interactions are typical of local neuronal circuits and may occur through the use of serial synaptic arrays (such as mammalian dendro-dendritic synapses). Little is known about GST and less about GST as it occurs in spiking neurons. Many cells involved in local neuronal circuits in the major invertebrate phyla and in mammals (including man) probably use GST as a major method of synaptic communication. This research should be a first step toward understanding how these neurons function.